Ethylene is typically produced by a process referred to as steam cracking, wherein a hydrocarbon feedstock is converted into an ethylene-containing cracked gas product. Ethylene is produced by cracking a feed mixture of dilution steam and hydrocarbon feedstock in the radiant zone of a cracking furnace. The feed mixture is preheated in the convection zone of the cracking furnace, wherein it is in heat exchange contact with flue gasses from the radiant zone, thereby recovering heat from the flue gas, and subsequently enters the radiant zone. The cracked gas product comes out of the radiant zone of the cracking furnace at elevated temperatures and is cooled against water in a heat exchanger, typically referred to as an indirect quench exchanger (IQE which are sometime called transfer line exchangers (TLE or TLX), selective linear exchangers (SLE), primary, secondary, tertiary, etc. quench exchangers (PQE/SQE/TQE/etc.), or ultra-selective exchangers (USX)). Saturated steam is produced in the IQE by quenching the cracked gas product.
The cracked gas product is subsequently separated in a separation process, which includes one or more compression and refrigeration steps.
In a conventional steam cracking unit, also referred to an ethylene cracker unit, steam expansion turbines are used to drive the cracked gas compressor and the refrigeration compressors. The required steam is generated in the in the indirect quench exchangers (IQE) as well as boiler coils in the furnaces and heaters.
In US 2009/0158737, a process for recovering power from steam cracking processes is described. In the process of US 2009/0158737, a multistage expansion turbine system is proposed to increase the efficiency to the power recovery. In US 2009/0158737, two turbine stages are provided. Medium pressure steam exiting the first stage is reheated by heat exchange against high pressure steam provided to the first turbine stage and the reheated medium pressure steam is provided to the second turbine stage. The heat required to reheat the medium pressure steam is provided by superheating the high pressure steam retrieved from the IQE in the convection section of the cracking furnace to temperatures above those normally required to superheat the high pressure steam for purposes of driving a steam turbine.
A disadvantage of the process of US 2009/0158737 is that steam cracking furnaces including utilities provided for handling the high pressure steam are typically operated at their maximum design limits. In order to allow the handling of superheated steam at temperatures normally used, special alloys are employed for piping and heat exchanger. In the process of US 2009/0158737, the superheated steam is heated to even higher temperatures, either requiring the use of alloys that can handle the higher temperatures resulting in increased CAPEX or accepting a significantly shorter lifetime of the heat exchangers and piping. Moreover, the process of US 2009/0158737 requires multiple heat exchange steps, leading to loss of energy efficiency and increased CAPEX. At least one of these heat exchange steps will include a heat exchange between two streams having a significant pressure difference. As described in Example 2 (Example of the Present Invention, with reference to FIGS. 2 and 3,) of US 2009/0158737 the superheated high pressure steam 30 (70 in FIG. 3) is heat exchanged with a medium pressure stream 37 (76 in FIG. 3), whereby the pressure difference between stream 30 and 37 (70 and 76 in FIG. 3) is more than 80 bar. Such a high pressure difference puts significant restriction on the design and material choice of the heat exchanger in particular at temperature over 585° C., leading to increased CAPEX.
There is a need in the art for a more efficient process for power recovery in a process for producing ethylene.